Ethernet is a shared local area (LAN) networking technology that was developed in the early 1970s. The basic design includes a shared transmission medium such as coaxial cable and optical fiber. Since the communication medium is shared, nodes listen to make sure that the cable is not in use before transmitting data. Ethernet also defines protocols to handle collisions that occur when two nodes transmit data simultaneously.
Ethernet switches are Layer 2 devices that provide a switching matrix or fabric that can temporarily connect a port to any other port. For example, a computer A is connected to port A of a switch. A computer D is connected to port D of the switch. The switch connects the ports A and D and provides a contentionless, full-bandwidth link to the computers A and D. Since the computers A and D are the only devices that are connected to the link, they are the only devices that compete for the link. The switch forwards frames or packets from one port to another—unlike a hub that forwards frames or packets to all other ports. This reduces traffic and increases security. A group of computers or nodes may also share a single port of the switch. Multilayer switches combine routing functions with switching. Other switches provide half and full duplex modes. The full duplex mode allows systems to establish connections and to send and receive data across a separate twisted pair or other media.
Gigabit Ethernet networks provide transmission speeds of 1000 Mb/s and include modes such as 1000Base-X and 1000Base-T. 1000Base-X modes include 1000Base-LX (IEEE 802.3z) and 1000Base SX (IEEE 802.3z). 1000Base-LX implements long-wavelength (1310 nm) laser transmissions with links up to 550 meters over multimode fiber optic cable and 3000 meters over single mode fiber optic cable. 1000Base SX implements short-wavelength (850 nm) laser transmissions over multimode fiber optic cable. 1000Base-T provides transmissions over four pairs of category 5 cable with a maximum distance of 100 meters per station to a switch or 205 meters end to end.
Referring now to FIG. 1, a flexible Ethernet switch 10 allows selection of the type of physical media that is used. The switch 10 includes a plurality of ports 12-1, 12-2, . . . , and 12-n. Some of the ports such as ports 12-1, 12-2, 12-3, and 12-4 are fixed media ports. Other ports such as ports 12-5 and 12-6 are configurable media ports. Gigabit interface connector (GBIC) modules 16-1 and 16-2 are connected to one of the configurable ports 12-5 and/or 12-6. Since both sides (20-1 and 22-1 and 20-2 and 22-2) of the GBIC modules 16-1 and 16-2 are 1000BASE-X, autonegotiation information can be passed freely to physical layers 24-1 and 24-2 of devices 26-1 and 26-2 that are connected thereto, respectively.
Referring now to FIG. 2, for purposes of clarity reference numbers from FIG. 1 have been used in FIG. 2 to identify similar elements. A GBIC module 40 includes a physical layer with an integrated serializer/deserializer (SERDES). The GBIC module 40 connects 1000BASE-X media 20-1 and 20-2 from the switch 10 to 1000BASE-T media 42-1 and 42-2. The 1000Base-T media 42-1 and 42-2, in turn, is connected to physical layers 44-1 and 44-2 of devices 46-1 and 46-2, respectively. For example, see U.S. patent application Ser. No. 09/501,556, filed Feb. 9, 2000 and assigned to the assignee of the present invention, which is hereby incorporated by reference. Since there is no direct path between 1000BASE-X and 1000BASE-T networks for passing autonegotiation information, 1000BASE-X autonegotiation is disabled.
1000BASE-X autonegotiation information is exchanged by using special code groups that are not used during normal packet transmission. In 1000BASE-X autonegotiation, two devices (P and Q) communicate with each other over a link. The device Q is the link partner of the device P and the device P is the link partner of the device Q. 1000BASE-X autonegotiation uses the underlying media to pass 16 bits of autonegotiation information at a time. The 16 bits of autonegotiation information are embedded in configuration ordered sets that are not used during normal data transmission. Therefore, devices are able to distinguish whether the transmitted data is a normal packet or autonegotiation data.
When the device P transmits a first configuration ordered set with all 16 bits set equal to zeros, the link partner Q knows that the device P is restarting autonegotiation. The device P continues to transmit the first configuration ordered set with all zeros until it is ready to start autonegotiation. Once the device P is ready to start autonegotiation, it transmits a second configuration ordered set with 16 bits of autonegotiation data. At this point, the second configuration ordered set is not all zeros. The device P continues to transmit the second configuration ordered set until the link partner Q transmits a second configuration ordered set that is not all zeros. When both of the devices P and Q are transmitting the second configuration ordered sets, autonegotiation continues according to the IEEE protocol for exchanging data.
There are currently 6 bits of the second configuration ordered set that are defined for 1000BASE-X media. These bits indicate 1000BASE-X full duplex, 1000BASE-X half duplex, pause, asymmetric pause, remote fault 1 and remote fault 2. A device advertises these capabilities by either setting or clearing the defined bits. The device may advertise that it has the function only if it can actually perform that function. However, the functionally capable device may choose to not advertise the function.